1. Field of the Invention
The present invention relates generally to pneumatic test equipment and more particularly to apparatus for providing precision pressure reference sources especially applicable as test and maintenance apparatus particularly useful in the testing of aircraft pneumatic avionic equipment.
2. Description of the Prior Art
Pressure reference sources have been known and used as laboratory and factory test tools as well as for testing and maintaining pneumatic systems such as air data measurement and display systems for aircraft. In the past, many of these systems have relied on manometric devices involving analog servo systems subject to undesirable lag, amplifier drifts, and offsets and difficult to isolate completely from ambient temperature and pressure conditions. Being fragile and large in size, they are generally restricted to use as permanent fixtures in stationary locations.
There has also been employed in the past a pressure reference system, similar in some respects to the present system, in which an electrically-controlled valve is used to establish a desired reference pressure as a predetermined ratio between a predetermined high pressure and a vacuum. The reference pressure amplitude is detected by a pressure sensor, is converted into an electric signal, and is fed back for comparison with a signal representing the desired pressure, the difference controlling the servo valve to reduce the difference to zero. This single closed loop system is wholly analog and depends on phase comparison of the desired reference pressure signal and the sensor signal and is therefore subject to the drift, offset, and other errors normally associated with analog servo loops.
In a more recent system disclosed in U.S. Pat. 3,794,070 for a "Precision Reference Pressure Supply System", issued to Klem et al on Feb. 26, 1974 and also assigned to the present assignee, a closed-loop, precision variable pressure reference test system employs the same general type of single closed-loop precision valve and vibratory diaphragm pressure sensor arrangement. The arrangement of the Klem et al patent uses a single control loop system and is relatively precise and rapid in operation. However, in Klem et al, the commands and displays are in meaningless numbers, hence primarily useful only in automatic test equipment not involving selectable data in conventional units or dimensions. Also, the single loop system is not highly responsive and tends to overshoot the commanded pressure value and its response is sluggish. Further, the single loop system of Klem et al did not provide precision closed-loop pressure rate control nor did it provide Q.sub.C control. Finally, offsets in the Klem et al system required periodic adjustments to maintain equality between the commanded and displayed pressures.